couple of things to check out.
1) Vacuum leak - vac. hoses, intake manifold gaskets, intake hoses, crankcase vent valve
2) Catalytic converter(s)
My bet is that you need to replace the crankcase ventvalve and do a complete intake manifold reseal. Do you ever hear a honking or groaning type of sound? One easy way to test for a bad crankcase vent valve is by letting the car idle in park, raise your hood and pull the engine oil dipstick. Many times this will cause the valve to start making noises.
However my second bet (a very close second) is one or both of your catalytic converters. with 86K miles on the clock it wouldn't surprise me either.
There could even be a chance that both are the culprit or neither are.
check this link out http://www.bmw-forums.com/forum/inde...showtopic=9451
its a post in this same forum about a 540 6spd
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